Conventional sintered silicon nitride is known to have a composition, for example, of silicon nitride/rare-earth oxide/aluminum oxide, or silicon nitride/yttrium oxide/aluminum oxide/aluminum nitride/titanium. A sintering aid, including a rare-earth oxide such as yttrium oxide (Y2O3) in the composition described above, has widely been used to enhance sintering, thus increasing the density and the strength of a sintered body.
Conventional silicon nitride sintered body for use in a rolling bearing member, which requires high abrasion resistance (wear resistance), in particular excellent sliding characteristics, is commonly manufactured using as a raw powder a high-purity silicon nitride fine powder, for example, synthesized by thermal decomposition of an imide.
However, the conventional silicon nitride sintered body for use in a rolling bearing member is manufactured by using an expensive raw powder synthesized in accordance with a process of thermal decomposition of an imide. Furthermore, the conventional silicon nitride sintered body has so high a mechanical strength or a fracture toughness that the sintered body has poor workability. Thus, there has been posed a problem that a product formed of the abrasion resistant member involves higher manufacturing costs.
Although the silicon nitride sintered body manufactured by the conventional method has a higher bending strength, increased fracture toughness, and improved abrasion resistance, it is insufficient in rolling characteristics and durability particularly necessary to the bearing member. Thus, the silicon nitride sintered body needs further improvement.
In recent years, there has been an increase in demand for ceramic material as a member for a precision apparatus. In such an application, the advantages of ceramics of high hardness, light weight (lightness), and high abrasion resistance are utilized together with high corrosion resistance and low thermal expansion. In particular, the application of the ceramic material as an abrasion resistant member forming a slide portion, such as a bearing, grows rapidly owing to its high hardness and excellent abrasion resistance.
However, when a bearing rolling ball is made of a ceramic abrasion resistant member, the abrasion resistant member is insufficient in the rolling life. Repeated rolling of the rolling ball under high stress in contact with a casing may cause exfoliation or fracture (crack) within a short period of operation. The exfoliation or fracture develops vibration of an apparatus including the bearing or tends to cause trouble damaging the apparatus. In either case, there has been also posed a problem that the ceramic abrasion resistant member exhibits poor durability and low reliability as a component material for the apparatus.
Further, manufacture of an abrasion resistant member having a fine uniform sintered compact structure and excellent strength characteristics requires a high-purity ceramic raw material with less impurity content. This situation increases the raw material cost and thus greatly increases the manufacturing cost of the abrasion resistant member.
The present invention had been achieved to address the problems described above. Accordingly, it is an object of the present invention to provide a silicon nitride abrasion resistant member suitable for a rolling bearing member and a method for manufacturing the member. The silicon nitride abrasion resistant member, even when prepared from inexpensive powdered silicon nitride manufactured by metal nitriding, has a mechanical strength, abrasion resistance, and a rolling life, equal to or higher than those of conventional sintered silicon nitride, and has particularly excellent workability.